The present invention relates to combustion systems, particularly to increasing the efficiency of combustion systems while simultaneously reducing polluting exhaust emissions, and more particularly to a combustion system which combines multi-stage combustion technology with nitrogen-enriched air technology.
Increasing fuel efficiency in combustion engines while simultaneously reducing polluting exhaust emissions has been researched over the past 25 years and subsidized by the Federal Government. Maximum fuel efficiency normally occurs at or near stoichiometric conditions where the fuel is completely oxidized. In practice, the combustion process in an engine is usually with air and not with pure oxygen. When oxygen is supplied by dry air, 3.76 moles of nitrogen will accompany one mole of oxygen. The stoichiometric air-fuel ratio is the ratio of the mass of air to the mass of fuel to result in stoichiometric combustion. The actual operating condition of an engine is usually expressed in terms of the equivalence ratio, which is the ratio of the stoichiometric air-fuel ratio to the actual air-fuel ratio. The equivalence ratio is 1.0 when the engine is operating at stoichiometric conditions. When an engine operates at an equivalence ratio greater than 1.0, it is operating fuel rich and produces pollutants such as hydrocarbons (HC), carbon monoxide (CO) and particulate matter. At equivalence ratios less than 1.0, the engine produces oxides of nitrogen (NOx) which is major source of photochemical smog and is regulated. Also, the combustion gases can be very corrosive with the excess oxygen and reduce the life of the engine.
Past research developed efficient engines by operating with equivalence ratios less than one, but will not meet impending requirements for greatly reduced NOx. Current research includes the development or modification of engines such as Variable Compression, Homogenous Charged Compression Ignition (HCCI), Clean Energy System (CES), Gasoline Injection and nitrogen augmentation or enriched air combustion. Emissions can also be reduced using catalytic converters for spark-ignited engines or using the Plasma-Assisted Catalytic Reduction (PACER) process for lean-burn engine.
By increasing the combustion temperature and pressure, the efficiency of a combustion system can usually be increased. As shown in FIG. 1, the efficiency of the Brayton cycle generally increases with temperature and pressure, but can decrease with pressure when the temperature is too low. Hence, it is important to operate at the highest temperature as possible to obtain high efficiency. The maximum operational temperature of most combustion systems, especially continuous flow ones, is limited by the materials of construction and the corrosive and oxidative products of combustion. In general, higher operational temperatures decrease the materials physical properties (e.g., strength) and increase corrosion and oxidation of the material.
Typically, gas turbines operate with excess air ("PHgr"=0.4-0.7) to reduce the operational temperature but produces large amounts of corrosive and oxidative gases. A two-staged turbine can be used to reduce the corrosive and oxidative gases by burning fuel rich in the first stage and stoichiometrically in the second stage. However, in order to obtain acceptable combustion temperatures in the first stage, the fuel ratio tends to be excessively high which can result in soot formation and fouling.
Another approach to reduce the combustion temperature is to use nitrogen augmentation or enriched air. Air separation technologies are used to enrich nitrogen in air up to 100%. To obtain reasonably high efficiency, the fuel and nitrogen enriched air are combusted in a single stage at stoichiometric conditions which requires relatively large amounts of highly enriched nitrogen oxidant.
The problem of increasing fuel efficiency while reducing emissions should be solved because it affects energy consumption and the environment. The present invention provides a solution to this problem wherein multi-stage combustion technology is combined with nitrogen-enriched air technology. In the combustion system of the present invention, the first stage of combustion operates fuel rich where most of the heat of combustion is released by burning the fuel with nitrogen-enriched air. Part of the energy in the combustion gases is used to perform work or to provide heat. The cooled combustion gases are reheated by additional stages of combustion until the last stage is at or near stoichiometric conditions. Additional energy is extracted from each stage to result in relatively high thermal cycle efficiency.
It is an object of the present invention to increase fuel efficiency in combustion systems while simultaneously reducing polluting exhaust emissions.
A further object of the invention is to provide a combustion system which controls the combustion temperature and products so as to extend the maintenance and lifetime cycles of materials in contact with the combustion products and reduce pollutants while maintaining relatively high combustion and thermal cycle efficiencies.
Another object of the invention is to combine multi-stage combustion technology with nitrogen-enriched air technology.
Another object of the invention is to provide a multi-stage combustion system using nitrogen-enriched air.
Another object of the invention is to provide a multi-stage combustion system wherein the first stage of combustion operates fuel rich, wherein part of the energy in the combustion gases is used to perform work or to provide heat, and wherein the cooled combustion gases are reheated by at least one additional stage of combustion until the last stage is at or near stoichiometric conditions.
Other objects and advantages of the present invention will become apparent from the following description and accompanying drawing. The invention is a multi-stage combustion system using nitrogen-enriched air. The combustion system of this invention can increase fuel efficiency while simultaneously reducing polluting exhaust emissions. For example, a two-stage turbine can be used to reduce the corrosive and oxidative gases by burning fuel rich in the first stage and stoichiometrically in the second stage, with the combustion temperature being reduced by the use of nitrogen augmentation or enriched air. By operating the first stage of combustion by burning the fuel with nitrogen-enriched air, most of the heat of combustion is released in the burning process, part of the energy in the combustion gases being used to perform work or to provide heat, and the cooled combustion gases are reheated by additional stages of combustion until the last stage is at or near stoichiometric conditions. Additional energy is extracted from each stage to result in relatively high thermal cycle efficiency. The air is enriched with nitrogen using air separation technologies such as diffusion, premeable membrane, absorption, and cryogenics. The invention has application for various types of combustion systems including turbines, boilers, furnaces, as well as combustion engines, and various types of fuels can be utilized including hydrogen, coal, methane, and various other carbon-based fuels.